Going back to the discussion of movie casting, or hiring practices in general, historical context is important to note.
For 200 years, black people were a largely enslaved population in this country. And even after they were freed as a culture, they still faced a further hundred years of subservience prior to the civil rights act. Now, let's pretend for a moment that the civil rights act fixed all the problems with racism in our society and the people who fought against the civil rights act (and all the practices that still existed) just disappeared or calmly said "we've been beaten, we'll change our ways now". That still leaves hundreds of years in which white people were able to make tremendous gains while black people were not.
Imagine a company was founded in 1900 by a white businessman. That company likely wouldn't have employed many black workers, if any at all. The company grows and grows, but likely maintains the same hiring practices. Then the civil rights act happens. Equality is legally enshrined in our country. That company would still have all white executives (or an all white board or directors), and a predominantly white workforce. So even with the new equality between the races, imagine that this company has a job opening. A black man applies and a white man applies, both with relatively equal qualifications. The white man is more likely to get the job, not necessarily because of overt racism, but because of intangible qualities. Psychologically, people are more likely to relate to those similar to them, so if you look at a predominantly-white workforce, you then turn to the white applicant and think "this guy will fit in better with our current workplace culture." Plus, there's the added drawback for black applicants in this scenario is that they are less likely to have someone already within the company (be it a friend or family member) to vouch for them or refer them for the position.
This all compounds and furthers the disparity in employment at the company between black and white employees. The reason diversity quotas exist is because without them, it's possible that the "best" employee (as you say) for the job might be overlooked and outright snubbed, if he's not white.
So, what do you suggest? Affirmative action? (is this the correct term? I think this is the correct term) In this case, sure, it help the minority, but it also hurt the "strong". Is this the right course of action? does the "strong" have a right to complain about this? I don't think there is a easy answer for this. I can't picture a scenario where everyone wins and everyone are happy. Some will have to get hurt in favor of others. I am just asking what would be the best course of action, in which nobody gets hurt?